1. Field of the Invention
The present invention relates to a bypass air flow rate control apparatus for an internal combustion engine and, more particularly, to an apparatus for controlling to change the operation of a plurality of control valves provided in bypass passages.
2. Description of the Related Art
Japanese Patent Unexamined Publication No. 2-308942 discloses an idle speed control apparatus which maintains engine speed at a desired level despite aging deterioration of an idle speed control valve for controlling air flow rate or a reduction in air density (or air pressure) occurring, for example, at a high altitude. This apparatus comprises first and second bypass passages bypassing a throttle valve. The first bypass passage is provided with an idle speed control valve, and the second bypass passage is provided with an electromagnetic on-off control valve. The opening degree of the idle speed control valve is continuously adjusted in accordance with duty values.
In order to prevent for a required amount of intake air from being not obtained due to aging deterioration of the idle speed control valve or a reduction in air density caused at a high altitude, the above-described apparatus has preset upper and lower critical values for duty values. When a duty value exceeds the upper critical value, the on-off control valve is applied an electric current and thus opened, and, simultaneously, the duty value for the idle speed control valve is reduced by an amount corresponding to the air flow rate passing through the on-off control valve. Whereby, the opening degree of the idle speed control valve is reduced by an amount corresponding to the air flow rate passing through the second bypass passage, so that it is possible to increase control region of the idle speed control valve when air flow rate is required.
When the a duty value becomes lower than the lower critical value and the on-off control valve has been opened, the duty value for the idle speed control valve is increased and, simultaneously, the on-off control valve is closed.
As described above, an amount of intake air corresponding to duty value is always obtained by the switch over control of the idle speed control valve and the on-off control valve.
However, there is a case in which the characteristics of the air flow rate passing through the idle speed control valve and the on-off control valve are varied due to aging deterioration, product variation and the like. Therefore, when the on-off control valve is opened by an electric current being applied and the opening degree of the idle speed control valve is reduced by an amount corresponding to the increased air flow rate of the on-off control valve, there is a case in which the bypass air flow rate varies instantaneously for example, as shown in FIGS. 5 and 6.
FIGS. 5(a) and 6(a) show the relationships between air flow rate passing through the idle speed control valve and time, FIGS. 5(b) and 6(b) show the relationships between air flow rate passing through the on-off control valve and time, and FIGS. 5(c) and 6(c) show the relationship between bypass air flow rate and time.
As shown in FIGS. 5 and 6, when the idle speed control valve and the on-off control valve are switched over at time T, the bypass air flow rate is temporarily increased or reduced if the valve opening rate and the valve closing rate of the valves are different from each other even if air flow rates changed by the opening and closing of each of the valves are identical with each other.
Further, in the apparatus of the publication, the switch over from the idle speed control valve to the on-off control valve is effected when a duty value for the idle speed control valve is beyond the upper or lower critical value. Therefore, during idling, the change in the bypass air flow rate causes a relatively large torque change, thus causing a shock to the vehicle and the driver to have uneasy feelings.